A substantial body of literature exists with respect to the catalytic hydrogenation of aromatic amines to prepare the corresponding cycloaliphatic amines. Illustrative of this type of reaction is the hydrogenation of methylenedianiline [4,4'-diaminodiphenylmethane, MDA] to the corresponding cycloaliphatic amine, bis (4-aminocyclohexyl)methane [H.sub.12 MDA, PACM].
The hydrogenation follows a step-wise reaction sequence, giving first the half hydrogenated cis and trans isomers of p-(4-aminocyclohexylmethyl)aniline, [4-(p-aminobenzyl)aminocyclohexane, H.sub.6 MDA], then reacting further to yield the three bis(4-aminocyclohexyl)methane isomers (cis, cis; cis, trans; and trans, trans) represented by the formulas and reactions as follows: ##STR1##
Some of the early hydrogenation work to produce bis(4-aminocyclohexyl)methanes was done by Whitman and Barkdoll, et al and their work is set forth in a series of U.S. Pat. Nos. 2,511,028, 2,606,924, 2,606,925 and 2,606,928. Basically the processes described in these patents involve the hydrogenation of methylenedianiline at pressures in excess of 200 psig, preferably in excess of 1,000 psig at temperatures within a range of 80.degree. to 275.degree. C. The hydrogenation is carried out under liquid phase conditions and an inert organic solvent is used. Most of the references utilize a noble metal catalyst such as ruthenium, rhodium, iridium, or mixtures of any of these or with platinum or palladium, either as the hydroxide, oxide, or the metal itself on an inert support. Examples of ruthenium catalysts utilized for the hydrogenation process include ruthenium oxides, such as ruthenium sesquioxide and ruthenium dioxide; ruthenium hydroxide; and ruthenium salts.
The isomeric cycloaliphatic diamines are useful in the preparation of the corresponding aliphatic diisocyanates suitable for forming light stable urethane coatings and lacquers. In earlier experiments involving the hydrogenation of aniline, it was shown that addition of ammonia not only suppresses by-product formation mainly from hydrogenolysis and condensation reactions, but also poisons the catalyst. However, addition of lithium hydroxide and sometimes sodium hydroxide suppresses the hydrogenolysis without the detrimental poisoning of the catalyst. A similar phenomenon has been reported with the hydrogenation of methylenedianiline using lithium hydroxide, and to a lesser extent, with other alkali or alkaline earth hydroxides or alkoxides. Common by-products formed during the hydrogenation of methylenedianiline include the hydrogenolysis products 4-aminodicyclohexylmethane and 4-aminocyclohexylcyclohexenylmethane, the hydrolysis product 4-amino-4'-hydroxydicyclohexylmethane, and higher boilers, mainly higher molecular weight secondary amine condensation products. All of these products exist as a number of isomers.
In the continued development of processes for manufacturing bis(4-aminocyclohexyl)methanes by hydrogenating methylenedianiline it was found that if the ruthenium was loaded upon a support and the support was alkali-moderated, the catalyst was much more active and catalytically effective in producing the desired hydrogenated bis(4-aminocyclohexyl) methane product. Alkali moderation was effected by contacting the catalyst with an alkali metal hydroxide or alkoxide; also, such alkali moderation of the catalyst could be effected prior to hydrogenation or in situ during the hydrogenation. Representative patents showing the utilization of alkali moderated ruthenium catalysts to hydrogenate methylenedianiline include U.S. Pat. Nos. 3,636,108, 3,644,522 and 3,697,449. Alkali metal and alkaline earth metal nitrates and sulfates have similarly been shown effective in U.S. Pat. No. 4,448,995 under high pressure (4000 psig) hydrogenation conditions.
U.S. Pat. No. 3,959,374 discloses a process for the preparation of bis(4-aminocyclohexyl)methane by pretreating a mixed methylenedianiline system with a nickel-containing hydrogenation catalyst prior to hydrogenation with ruthenium. The pretreatment was used to overcome low yields (52.4%) and long reaction time associated with nickel and cobalt catalysts. Ruthenium catalysts, although commonly used for hydrogenation, were not suited for hydrogenation of a feed containing impurities according to the teachings of the '374 patent, since impurities in the feed caused a rapid decline in activity and hydrogenation efficiency.
U.S. Pat. Nos. 3,347,917; 3,711,550; 3,679,746; 3,155,724; 3,766,272 and British Patent No. 1,122,609 disclose various isomerization and hydrogenation processes to produce bis(4-aminocyclohexyl)methane containing a high trans,trans-isomer content; i.e. an isomer content near equilibrium typically 50% trans,trans, 43% cis,trans and 7% cis,cis. Ruthenium catalysts were used to effect isomerization.
In U.S. Pat. Nos. 4,394,522 and 4,394,523, processes are disclosed for producing bis(4-aminocyclohexyl)methane by carrying out the hydrogenation of methylenedianiline in the presence of unsupported ruthenium dioxide at pressures of at least 2500 psig or in the presence of ruthenium on alumina under pressures of at least 500 psig and preferably from 1500 psig to 4000 psig in the presence of an aliphatic alcohol and ammonia. Other catalysts have been utilized for the hydrogenation of methylenedianiline and examples are shown in U.S. Pat. Nos. 3,591,635 and 3,856,862 which disclose the use of a rhodium component as a catalytic material and each require the use of an alcohol as a solvent. The rhodium is alkali moderated using ammonium hydroxide as a pretreatment or by carrying out the reaction in the presence of ammonia.
Although different organic solvents and mixtures have been used (generally ethers and alcohols), aqueous systems have generally not been utilized. U.S. Pat. No. 4,448,995 teaches that this reaction should be maintained in an anhydrous state or at least maintained so that water concentration is less than 0.5% by weight because failure to do so results in an increase in both the amount of alcohol by-products and higher molecular weight condensation products. In addition, the patent states that alkali nitrates and sulfates, especially those of lithium reduce by-products.
U.S. Pat. Nos. 4,960,491 and 4,754,070 disclose the hydrogenation of aromatic amines to their hydrogenated counterparts. In these patents, it is stated that "Although in some processes water can be used as a co-solvent, it is preferred that the system be in an anhydrous state or at least maintained so that the water concentration is less than 0.5 percent by weight. Water, when present in the system, tends to increase the amount of by-product alcohols and heavy condensation products during the hydrogenation process and tends to deactivate the catalyst system."
Representative supports for ruthenium catalysts for the hydrogenation of aromatic amines in aqueous alkaline media are disclosed in U.S. Pat. No. 3,697,449, including bauxite, periclase, zirconia, titania, and diatomaceous earth. The '449 patent discloses a series of hydrogenation reactions of methylenedianiline using a sodium hydroxide promoter while varying the amount of water employed in the reaction mixture, although disclosing no advantages associated with the use of water, and instead teaching that "excessive quantities" of water above about 5 weight percent causes unwanted by-product formation. More specifically, it is disclosed in the working examples of the '449 patent that increased water in the hydrogenation reaction mixture lowers the yield of product, and it is stated in claim 1 "that the maximum amount of water that can be present in the reaction medium is about 5 weight percent of said medium". Moreover, the '449 patent discloses, at column 1, lines 56-60 thereof, that the hydrogenation of aromatic amines in aqueous alkaline systems in the presence of more than about 5% water in the medium results in the formation of undesirable byproducts. The '449 patent also states that "if the basic alkali metal compound is added to the hydrogenation process as an aqueous slurry or solution, the amount of water added should be no more than about 5% by weight of the reaction mixture. Excessive quantities of water will normally result in the formation of undesirable by-products, including the corresponding hydroxy derivatives, deamination products, and polyamines."
In some comparisons, the presence of lithium hydroxide has been shown to actually result in an increase in the production of higher molecular weight products. (See U.S. Pat. No. 4,946,998.)
New processes for the hydrogenation of aromatic amines that provide a rapid rate of reaction without appreciable by-product production would be highly desired by the saturated cyclic amines manufacturing community. The present invention provides one such process.